An accumulator (a common rail type) fuel injection apparatus is constructed to implement pumping of fuel under a pressure to a common accumulation chamber by means of a high pressure fuel feed pump driven by an engine and to allow a fuel injection nozzle of each cylinder to be connected to the accumulation chamber, so as to inject the high pressure fuel stored in the accumulation chamber to each cylinder of an internal combustion engine.
The fuel injection amount to each cylinder is uniquely determined by a pressure prevailing in the accumulation chamber, i.e., an accumulation chamber pressure, and an electric conduction time for which electricity to the fuel injection nozzles (injectors) provided for the respective cylinders lasts.
Accordingly, accurate control of the accumulation chamber pressure enables to achieve highly precise fuel injection control.
In general, fuel pumping control from a fuel feed pump to an accumulation chamber often has, as shown in FIG. 7, both a feedback control unit 01 and a feedforward control unit 02, and a feedforward amount is obtained in the feedforward control unit 02 from maps for each combination with a target pressure, a command value of fuel injection amount, and a number of engine revolutions.
Then, an output of the feedback control unit 01 and an output of the feedforward control unit 02 are added together and a pump discharge command value, for example an amount of plunger stroke as a pump discharge command value when a pump 03 is comprised of a plunger pump, is commanded to drive the pump 03 so as to feed the fuel to a common rail 04, and thus the pressure in the common rail 04 is controlled so as to be maintained at determined target pressure.
The maps used in the above-mentioned feedforward control unit 02 are often obtained by experiment in advance. Another technique also may be applied to obtain a feedforward amount from inverse characteristics of a formulation model of pump and common rail.
For example, techniques disclosed in Patent Document 1 (Japanese Laid-Open Patent Application No. 2005-76618) and Patent Document 2 (Japanese Laid-Open Patent Application No. 2005-301764) are known regarding pressure control in a common rail.
In this Patent Document 1, a technique of using both feedforward control and feedback control is disclosed, in which pressure in the common rail is equalized by repeating a procedure of, in correspondence with the crank angle of an engine, detecting fuel pressure in the common rail to calculate a difference from predetermined target fuel pressure, outputting a part of the pressure difference as a feedforward amount, applying feedback control to the rest, and adding the feedforward amount to an output of the feedback.
In addition, Patent Document 2 creates a dynamic model for a common rail system and calculates a control amount in association with a target fuel pressure based on the model to thereby execute the feedforward control.
Nevertheless, since a feedforward amount is determined by a combination of a target pressure, a command value of fuel injection amount, and a number of engine revolutions in the feedforward control unit 02 shown in FIG. 7, if a disturbance is developed that acts as an unexpected control variable other than the target pressure, the fuel injection amount, and a fluctuation in the number of engine revolutions, such a disturbance cannot be controlled for the reason that the disturbance is out of coverage for control. Therefore, the control performance must be deteriorated. Moreover, in case of creating multidimensional maps including control variables other than the target pressure, the command value of fuel injection amount, and the number of engine revolutions, there occurs a problem of increasing the number of test cases and thus requiring enormous amount of labor and efforts.
In addition, although the technique of Patent Document 1 complements a response delay of feedback control complemented with feedforward control using both the feedforward control unit 02 and the feedback control unit 01, such control is insufficient in a case that any unexpected disturbance is developed, and furthermore, the technique disclosed in Patent Document 2 does not exhibit sufficient control performance when a disturbance other than the conditions for creating the dynamic model of common rail system is developed.